What Is Bridge?

            In Local Area Network (LAN) Bridges supports the single broad cast domain and separate collision domain. Bridges are mostly used to connect two Local Area Networks (LAN) that uses same protocols. Bridges are intelligent devices than hub and can take different decisions related to message forwarding. Bridges is layer two devices and filters the traffic on the basis of MAC addresses. Bridges perform five basic operations learning, forwarding, flooding, filtering and ageing. Bridges can learn the addresses of networks and then maintain the learning table on order to forward right message to the right network.

            There are different types of bridges used in Local Area Network (LAN). Local bridges and remote bridges are two different types of bridges. Local bridges connect Local Area Network (LAN) in the same area. Remote bridges connect Local Area Network (LAN) in the different areas. There are two ways for bridging in Local Area Network (LAN).

            The first techniques used for bridging is transparent bridging or learning. This type of bridging is used in Ethernet Local Area Networks (LAN). Second technique is source route bridging which is used with token ring Local Area Networks (LAN). Sometime bridges are used where repeater cannot function.

          A device which is used to connect two LANs of the same protocol (for example, Ethernet or token ring) is known as bridge.Mapping from one Mac format (for example to interconnect an Ethernet or Token ring LAN) to another is possible by using a bridge.
You can say that bridge is used as an address filter because it picks a packet from one LAN and sends it to another destination LAN. It does not add anything to the packet or does not modify any contents of the packet.

          A bridge works at layer 2 of the OSI Model. A bridge must have addressing and routing intelligence and must know which addresses are on each packet network to know which frames to pass.

Advantages: (1) By using a bridge a large network can be managed effectively.
(2) We can use bridges with LANs supporting different protocol stacks.
(3) AS by using a bridge we portioned a LAN into many smaller segments so it improves the reliability, and availability of the total network.

Disadvantages: (1) A bridge causes of a n additional store-and-forward delay compared with a repeater.
(2) Each bridge must generate new frame check sequence to handle any errors introduced while frames are being relayed across a bridge will go undetected.

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